The present invention relates to a polyurethane prepolymer which is preparable from at least one polyisocyanate substituted symmetrically in respect of NCO groups and at least one polyisocyanate substituted asymmetrically in terms of NCO groups, at least one polyol having an OH functionality of more than 2.5 and at least one polyol having an OH functionality of less than 2.5, to a process for preparing such a polyurethane prepolymer, to an adhesive comprising at least one such polyurethane prepolymer, and to its use.
One-component polyurethane adhesives (1K PU adhesives) are enjoying ever more frequent application, since they have considerable advantages over polyvinyl acetate dispersions (PVAc dispersions) in respect of heat resistance and water resistance. Adhesives of this kind, which are used very often in the field of wood bonding, have the drawback that, because of their intrinsic color, the glue line is apparent. In the field of the furniture industry and of interior outfitting, and also in the industrial sector, where certain light-colored fashion woods such as limba, ash or maple, for example, are being employed with increasing frequency, there is a call for adhesives with which the glue line has a zero or only very low visibility.
Particularly affected by this are all multi-layer glued constructions where, for the shaped parts to be produced, a large number of laminates are glued together, and where glue lines which are apparent produce severe disruption to the overall visual impression. Although adhesives based on polyvinyl acetate, for example, are known for their color neutrality, their use may be restricted for a variety of reasons.
Where, for example, high heat resistance or water resistance in the bond is a requirement, the last-mentioned adhesives are of only limited suitability, and are not suitable for every requirement. PVAc dispersions in particular, on account of their high water content of approximately 50% or more, cannot generally be used for multi-layer glued constructions, since the water fraction retards drying for an uneconomically long time and often, owing to the thermoplastic properties of the adhesive polymers, sufficient dimensional stability of the bond is often ensured.
Similar problems also result fundamentally in the case of adhesive bonds which join different materials, examples being wood and paper, wood and textiles, wood and leather, and similar mixed materials.
In order to avoid the problems described with adhesives based on polyvinyl acetate, the use of adhesives based on polyurethanes is on the increase in the aforementioned areas. Polyurethanes, however, have to date likewise been of only limited suitability for use in the abovementioned fields. A problem affecting polyurethanes in general is that they have a not inconsiderable intrinsic coloration.
Although there are polyurethane adhesives which have only a low intrinsic color and are less perceptible in a glue line, such polyurethane adhesives often have other drawbacks. For instance, such adhesives generally have inadequate open times or else combine acceptable open times with an excessive cure time.
DE 44 26 130 A1 relates to reactive hotmelt systems containing isocyanate groups, based on hydroxy polyols and diphenylmethane diisocyanates, with a 2,4′-diphenylmethane diisocyanate content of at least 70% by weight. A problem affecting these compositions is the curing rate, which in practice is often too low.
EP 0 568 425 A1 relates to adhesive compositions based on polyurethane prepolymers, which are obtainable by reacting MDI with polyether polyols. The MDI grades used are not characterized any further with respect to their composition.
DE 199 35 489 A1 relates to an adhesive composition intended for finger joint gluing and containing up to 99.999% by weight of a prepolymer which is obtainable by reacting at least one isocyanate-reactive compound and at least one compound containing an isocyanate. A description is given, for example, of the use of 4,4′-MDI or of mixtures of 2,4′-MDI and 4,4′-MDI, the description, in the case of the use of mixtures, being exclusively of mixtures which contain the two isomers in equal proportions or of mixtures in which the proportion of unsymmetric isomer is predominant. Compositions of this kind, however, have the disadvantages already mentioned above.
There was therefore a need for light-colored, storage stable polyurethane adhesives which cure rapidly within industry-standard processing times and which meet the requirements for water resistance and heat resistance.
In order to prepare PU adhesives with a light color it is necessary to use isocyanates which right from the start possess a light color. The so-called polymeric MDI is therefore suitable for such adhesives, since this compound has a dark intrinsic color. The polyisocyanate grades employed are therefore generally aromatic grades having a functionality of approximately 2.0. A problem affecting such adhesives based on difunctional isocyanates, however, is that their reactivity differs greatly from the reactivity of adhesives manufactured using polymeric MDI. Thus by using polymeric MDI it is possible to prepare prepolymers which exhibit a comparatively long open time with a relatively short curing time, whereas this pair of properties has so far been unavailable for aromatic polyisocyanate grades having a functionality of approximately 2.0.
The last-mentioned properties are, however, a fundamental requirement on the part of the user of such adhesives. In order to open up the applications already described above for polyurethane adhesives too, therefore, light-colored adhesives are required which at one and the same time exhibit the aforementioned properties in respect of processing time, cure time, water resistance and heat resistance. With the PU adhesives prepared to date on the basis of diisocyanates, however, it is found that either the processing time is too short or the cure time lasts too long. It is true that it is possible to influence the reactivity by appropriate adaptation of the formula: for example, by adapting the amount of catalyst used. However, for a given open time, the curing is then substantially slower or, for a given curing time, the open time is considerably shorter.
Additionally it is possible, using MDI grades including a 2,4-MDI fraction, to set satisfactory reactivity and color, but heat and water resistance in adhesives of this kind are often not adequate.
The object was therefore to provide an adhesive which no longer has the aforementioned drawbacks of the prior art. In particular the object underlying the invention was to provide an adhesive which combines a sufficient open time with short curing times. A further object underlying the invention was to provide an adhesive which exhibits the abovementioned properties and additionally has an essentially color-neutral effect on the bondline relative to the materials to be bonded.
It has now been found that PU adhesives which are obtainable from monomeric MDI grades with a 2,4-MDI fraction and polyols having an OH functionality of more than 2.5 have excellent properties in respect of open time, cure time, water resistance and heat resistance. The properties are comparable with those of adhesives based on polymeric MDI, with the color of a PU adhesive comprising monomeric MDI grades with a fraction of 2,4-MDI and polyols having an OH functionality of more than 2.5 being distinctly improved relative to those PU adhesives obtainable using polymeric MDI.